1. Field of the Invention
This invention relates to filler caps for tanks and, in particular, to an improved cap for permitting pressure relief without interference from a deformed sealing member.
2. Desirable Features of a Tank Cap
In providing a filler cap for a tank, such as the gasoline tank of an airplane, it is desirable that the cap be manually operable so that it may be removed when refueling is necessary and also securely locked into the tank opening after refueling without necessitating the use of specialized tools. It is further desirable that the filler cap be constructed in such a way whereby visual observation of the cap will readily show whether it is properly locked into the tank. In addition, these caps have to be constructed to meet rigid specifications and standards. They must be able to withstand any internal pressure created within the tank, must not vibrate loose during use, and where the tank will contain explosive fumes such as created by high octane gasoline, it is of utmost importance that the filler cap be lightning safe, that is, immune to electrical sparking on the fuel side of the point of seal without adversely affecting the sealing function of the cap.
3. History of the Art
In the past, filler caps of this general type have been constructed with a plug member for effecting a seal with the tank opening and a cooperating locking mechanism depending therefrom for locking the cap onto the tank in a sealed position. A shaft extending through the plug member is attached at its lower end to the locking mechanism and provided at its upper end with an actuating handle adapted to be received within a handle recess formed in the upper surface of the plug member. Movement of the handle into and out of the recess will cause longitudinal movement of the locking mechanism relative to the plug member while rotation thereof about the axis of the shaft will move the locking mechanism from a first position in which the cap may be readily removed from the tank opening to a second position where the locking mechanism may be moved toward the plug member to securely lock the cap in sealed relationship within the opening.
For receiving caps of this construction, the tank is provided with an adapter member which defines the tank opening. This adapter is constructed with an annular wall surface extending into the tank for receiving the plug member and an annular locking flange at the bottom of the wall surface. The annular flange is provided with openings or slots for permitting the locking mechanism to pass beyond it when the cap is inserted into the adapter so that the locking mechanism may then be rotated about the axis of the plug and into a position under the flange whereby movement of the locking mechanism toward the plug member will cause it to engage against the undersurface of the flange and lock the cap in sealed position within the tank opening.
With filler caps of this construction, the sealing of the plug member against the annular wall of the adapter member is effected by means of an O-ring fixed to the outer surface of the plug member. This O-ring extends beyond the outer periphery of the plug member and effects a spacing of the upper surface of the plug member from the surrounding adapter member and tank wall. This spacing does not prevent a proper sealing of the cap but does expose the sealing O-ring to the ambient atmosphere rendering it more susceptible to deterioration and swelling. Also, by having the exposed outer surface of the cap spaced from the surrounding adapter and outer wall of the tank, a situation is created in which an electrical charge striking directly against the cap or passing from the outer wall surface of the tank onto the cap may tend to more readily pass through the cap and inwardly into the tank rather than across the cap and back onto the outer surface of the tank. It is apparent that any electrical charge entering the tank and creating internal sparking, where, for example, the tank contains gasoline, may cause the tank to explode.
To overcome these problems, the filler cap disclosed in U.S. Pat. No. 3,391,817 is constructed so that its exposed upper surface seats directly upon or almost touches the adapter defining the tank opening. In addition, the sealing of the cap within the tank opening is effected by a compressible seal in the form of an O-ring positioned in a groove about the periphery of the plug member. As the cap is lowered into the tank opening, this O-ring is deformed into positive engagement with the opposed annular wall portion of the adapter and the flange of the plug member is seated directly upon the end of the annular wall of the adapter. To activate the seal member, it is positioned on the plug member of the cap in alignment with a slanted portion of the adapter member so that insertion of the cap into the opening will cause the sealing member to engage against the annular wall surface of the adapter at this slanted portion. Alternatively, a compression plate may be attached to the bottom of the plug member and slidably received over its lower end whereby upon locking of the cap into the opening, such compression member will be drawn into engagement with the O-ring to compress it and thereby cause it to deform outwardly of the plug member and into engagement with the annular wall surface of the adapter. In these caps, the O-ring is substantially free of restraint against radially outward deformation.
While these caps have performed well in many applications for many years, in some instances they have failed to relieve internal vapor pressures generated in the tanks. Typically the caps are removed from the tank by first lifting a lever arm to decompress the O-ring and thus destroy the seal. Then the lever is rotated to unlock the cap from the adapter. In some instances, however, the O-ring is deformed radially outward by either tank pressure or fuel induced swelling with the consequence that decompression does not relieve the tank pressure. If the cap is then unlocked, it can pop out with considerable force.
Accordingly, there is a need for an improved filler cap which can reliably relieve internal tank pressures without interference from the sealing member.